1. Field of Invention
The invention relates to a failure diagnosis apparatus and method of a fuel vapor purge system for purging fuel vapor generated in a fuel tank into an intake system.
2. Description of Related Art
A vehicle having a volatile liquid fuel tank commonly uses a so-called fuel vapor purge system. In a typical purge system, fuel vapor generated in a fuel tank is collected into a canister through a vapor passage. The fuel vapor thus collected is then purged into an intake passage of an internal combustion engine through a purge passage.
In order to ensure reliability of such a fuel vapor purge system, many purge systems incorporate a failure diagnosis apparatus for detecting leakage caused by a hole, damage, and the like in an evaporation route (which includes a fuel tank, a vapor passage, a canister and a purge passage). Whether there is such a leakage failure in the evaporation route can be determined by providing a pressure difference between the inside and the outside of the evaporation route, and detecting the behavior of the internal pressure of the evaporation route. The level of the internal pressure determined in a state where there is no leakage in the evaporation route is then compared with the detected behavior of the internal pressure.
Japanese Patent Laid-Open Publication No. 10-90107 proposes such a failure diagnosis apparatus of a fuel vapor purge system. This failure diagnosis apparatus pressurizes the inside of an evaporation route by delivering air under pressure into the evaporation route by an electric pump while an internal combustion engine is stopped. The failure diagnosis apparatus then determines whether there is a leakage failure in the evaporation route based on the behavior of the internal pressure of the evaporation route. A mechanical load on the electric pump varies according to the internal pressure of the evaporation route. Therefore, current consumption of the electric pump also varies accordingly. As such, the internal pressure of the evaporation route can be detected based on the current consumption of the electric pump. That is, if there is leakage in the evaporation route, the internal pressure of the evaporation route is less likely to vary. Therefore, the mechanical load on the electric pump does not increase from the beginning of the pressurization process, and current consumption of the electric pump remains low. In contrast, if there is no leakage in the evaporation route, the mechanical load on the electric pump increases as the evaporation route is pressurized. As a result, current consumption of the electric pump increases accordingly. In this way, whether there is a leakage failure in the evaporation route can be determined based on the current consumption of the electric pump in the pressurization process.
The electric pump degrades over time and detection accuracy of a leakage failure in the evaporation route declines because the pressurization capability of the electric pump with respect to the current consumption thereof is reduced. Therefore, in such a failure diagnosis apparatus, the electric pump is connected to a reference orifice having the same diameter as that of a hole to be detected as abnormal, and the reference orifice is pressurized by the electric pump. Current consumption of the electric pump during pressurization of the reference orifice is used as a reference level for abnormality detection. Reduction in detection accuracy can be prevented by comparing current consumption of the electric pump during pressurization of the evaporation route with the reference level.
In the failure diagnosis apparatus of the above publication, however, a route which is pressurized by the electric pump in order to detect a leakage failure is different from a route which is pressurized by the electric pump in order to pressurize the reference orifice. Accordingly, a pressure change caused by the fuel vapor generated in a fuel tank is included only when the internal pressure detects a leakage failure, and is not included when the reference orifice is pressurized. In other words, a current corresponding to the pressure change caused by the fuel vapor is included in the current consumption of the electric pump during a leakage failure diagnosis process. However, a current corresponding to the pressure change caused by the fuel vapor is not included in current consumption (reference level) of the electric pump during pressurization of the reference orifice. This results in reduced detection accuracy of a leakage failure in the evaporation route. It is therefore impossible to accurately detect a leakage failure.